Bearing arrangement

ABSTRACT

A bearing arrangement for an articulatable support ( 6 ) comprises bearing assemblies ( 20/28, 22/29, 24/30  and  26/32 ). In one embodiment the assemblies comprise a ball ( 20, 22, 24, 26 ) and aperture ( 40  FIG.  2 ) having sliding contact at discrete areas ( 36 ) during relative rotation. In another embodiment the sliding contact may be formed at a ball ( 20,22,24,26 ) and a plastics e.g. P.T.F.E. support ( 84, 85  FIGS.  10  and  11 ). Resilient displacement of the bearing contact along a rotational axis (A and B) is provided by planar spring supports ( 28, 32 ) and rigid support is provided by supports ( 29  and  30 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a bearing arrangement, particularly butnot exclusively in the field of precision bearings for example formeasurement apparatus having articulated wrist parts.

2. Description of Related Art

Precision bearings of many forms are known. The simplest rely on thesliding contact to transfer loadings. More complicated bearings haverolling contact, for example ball race or roller sets. Rolling contactis preferred for high loadings, high rotational speeds and long bearinglife. However these bearings need to be manufactured in an accuratemanner and are consequently expensive. Where thin section parts are usedthey tend to take up the shape of the housing to which they are fitted.As a consequence, their housings too must be accurately made if goodaccuracy of movement is to be maintained.

Sliding contact bearings may be used for precision bearings, for examplewatch bearings. Watch bearings, and similar, are lightly loaded and sodo not need large bearing surfaces to transfer forces. Typically arotatable shaft might have conical ends and be held in place between twoplates with complementary end accepting parts. This type of bearing,although simple and low cost, is difficult to adjust when wear takesplace. Wear being more common where sliding contact is employed.

One commercially available bearing, used typically for roundnessmeasurement machines provides a fixed block, e.g. of P.T.F.E., having aconical recess and a ball seated rotatably within the recess. Again wearadjustment for such a bearing is problematic and the arrangement is notused in pairs.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided abearing arrangement comprising:

-   -   two bearing assemblies each located on an axis;    -   each bearing assembly comprising two parts in contact during        their relative rotation;    -   at each assembly the contact taking place in a plane;    -   one of the assemblies allowing resilient displacement of its        contact plane and the other of the assemblies being relatively        rigid for preventing substantial displacement of its contact        plane.

Another well-known bearing is a ball joint. Typically a spherical partis held in a complementary socket for movement with more than one degreeof freedom. An example of such a joint is shown in FIGS. 8 and 9 ofEuropean Patent No. 680599 (Renishaw) see balls 80 and triangular socket78.

Each of the sockets 78 is formed in a leg 77 which is resilientlymovable. This means that tube 72 supported between two legs 77 may bedisplaced resiliently back and forth. In so doing these legs alter theposition of the rotational axis of the tube 72.

According to a second aspect of the invention there is provided abearing arrangement comprising:

-   -   two bearing assemblies each located on an axis;    -   each bearing assembly comprising two parts in contact during        their relative rotation;    -   at each assembly the contact taking place in a lane;    -   at least one of the assemblies allowing resilient displacement        of its contact plane in a direction parallel to the axis.

The shortcomings mentioned above are particularly noticeable when suchbearings are used for rotation (including pivoting) of elements ofmeasuring equipment. In such equipment exact repeatable circular motionis necessary even after bearing wear or thermal changes in dimensions ofcomponent parts, and lightness and low cost are desirable.

According to a third aspect of the invention there is providedmeasurement apparatus having a pivot including a bearing arrangementaccording to the first or second aspect of the invention. Preferably themeasurement apparatus includes a mechanical wrist element.

Preferred features of the invention according to the first, second orthird aspects are given in the subsequent paragraphs.

The two parts of each of the bearing assemblies may include a femalepart having a recess or aperture and a male part acceptable into therecess or aperture.

The contact may be sliding contact.

Where there is sliding contact this contact may be between the femaleand the male parts of at least one of the bearing assemblies and may beat discrete locations in the plane.

The discrete locations may be provided by a noncircular recess oraperture (e.g. triangular or trihedral) in the female part co-operatingwith a circular (for example spheroidal or conical) male part, or may beprovided by a circular (e.g. conical or straight-sided) recess oraperture in the female part co-operating with a non-circular (e.g.trihedral) male part.

Alternatively the contact may be rolling and the parts may include aball race.

The movement of the plane may be provided by a movable female part andwhere the movement is resilient the female part may be resilientlymovable. Possibly the female part is a planar support having springqualities.

Associated with each assembly may be first and second relativelyrotatable housings. The female and male parts of each assembly may beassociated with either of the first and second housings.

According to a fourth aspect of the invention there is provided asupport for a measurement probe comprising an articulatable wristproviding two axes of rotation for the probe, at a first axis therebeing provided a bearing arrangement as claimed in any one of claims 1to 10, the bearing arrangement being connected to a spindle having anextension extending beyond the bearing arrangement in the direction ofthe first axis.

The invention is described hereinafter with reference to measurementapparatus for use on a coordinate measuring machine (CMM). However, thisis an example of one possible application and is not intended to limitthe invention in any way. The invention could be applied to anyapparatus where relative rotation of parts is required e.g. a rotatablesupport for optical components which require accurate rotation possiblyof the type described in UK Patent Application No. GB 0019199.9.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawings,wherein:

FIG. 1 shows a probe, and an articulatable probe wrist employing theinvention;

FIG. 2 shows a detail of a component shown in FIG. 1;

FIG. 3 shows a sectional view through line III—III in FIG. 2;

FIG. 4 shows a further detail of a part of a component shown in FIG. 1;

FIG. 5 shows an alternative arrangement of some of the parts shown inFIG. 1;

FIGS. 6A-6F show various configurations of bearing arrangementsaccording to the invention;

FIG. 7 shows yet another alternative arrangement of some of the partsshown in FIG. 1; and

FIGS. 8 to 11 show alternatives to some of the parts shown in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1 there is shown a probe 2 having a stylus 4 forcontact with an item to be measured. Conventionally, in use, the quill 8of a CMM will be moved in any of the three orthogonal axes until thestylus 4 touches an item. The position of the quill 8 with respect tothe three axes is then used to determine dimensions of the item.

In addition to the three axes, a probe head 6, attached to the quill 8between the quill and the probe has a wrist configuration which adds tworotational axes of movement to the probe—rotation “AA” in a verticalplane and rotation “B” in a horizontal plane. Motors M_(A) and M_(B)provide the torque for the rotation in the two axes, so measurements maybe taken with or without moving the quill 8, by moving the probe head 6.

Articulation of the head allows more complex movements of the probe, forexample to measure the roundness of a bore along its length by movingthe stylus in a helical path around the bore. The position of the styluscan be calculated from the known rotational position of the stylus andthe three CMM axis readings.

Such rotational movement of the probe requires accurate and repeatablemovement of the bearings in the probe head. In this embodiment the probeis mounted to a carriage 14 held on a horizontal spindle 16. The spindle16 has a bearing assembly at each end connected to a lower housing 12 ofhead 6. The lower housing 12 is connected to a vertical spindle 18 in anupper housing 10 of the head 6 which has two bearing assemblies also.

Horizontal spindle 16 may rotate relative to the lower housing 12.Rotation is achieved by use of two bearing assemblies having parts 20and 28, and 22 and 29. Male bearing parts in the form of balls 20 and 22are fixed (for example by means of adhesive) to the ends of the spindle16. Each ball is accommodated in a female part in the form of supports28 and 29 each having male part accepting apertures therein such thatthe spindle's only possible movement is rotation “A”. Similarly lowerhousing 12 is rotatably mounted via vertical spindle 18. Two bearingassemblies are shown comprising male parts in the form of two balls 24and 26 accommodated in two female parts in the form of supports 30 and32 having apertures therein, all providing rotational movement “B”.

Balls 20,22,24 and 26 can be manufactured by known techniques to veryhigh accuracy, i.e. less than 0.16 microns overall roundness tolerance.This exact sphericity provides accurate movement of the probe inrotation. The balls may be of a ceramic, ruby or steel material.

Each spindle 16 and 18 has a fixed support i.e. 29 and 30 respectivelyand a movable support i.e. 28 and 32. The fixed support is held in fixedrelation to its mounting whereas the movable support can moveresiliently in the direction of the associated rotational axis.

Power and signal paths a,b,c,d and e are shown. Path a provides power tomotor M_(A) for rotating spindle 16 (and hence probe 2) about axis A. Inpractice this rotation will be pivoting motion in an arc of up to 180°.Path b provides power to motor M_(B) for rotating spindle 18 (and hencelower housing 12 and probe 2) about axis B. In practice this rotationcan be continuous so arcuate pivoting back and forth is not necessary.

The rotational position of spindles 16 and 18 can be determined byrotary encoders 86 and 88 respectively. Paths e and c are provided forthe encoder signals.

Paths a,d and e have a rotary coupling 90, the two halves of which maybe in sliding contact (e.g. by use of slip rings) or may be of thenon-contact type (e.g. a capacitive, inductive, infra-red, optical orR.F. link). The rotary coupling allows continuous rotation of the lowerhousing 12 relative to the upper housing 10.

FIG. 2 shows the form of the supports 28 and 29. Holes 34 for mountingscrews are shown, as is a central triangular aperture 40 for receiving aball 20 or 22. In order that the support 28 be resiliently movable it isformed from sheet, for example steel material or other planar materialwith inherent resilient spring-like qualities. Support 28 is arranged tobe deflectable under load, although, as described below, may bepreloaded at assembly also.

For simplification of manufacture support 29 can be identical to support28 but mounted rigidly (as illustrated) or a thicker material can beused. Alternatively support 29 might be integral with the lower housing12.

In this design three ball contacting areas 36 are formed on the threesides of the aperture 40. These areas are formed with a curved surfacehaving a radius larger than the radius of the ball 20 or 22. Areas 38 donot contact the ball. The contacting areas are formed by forcing a ball,of slightly larger diameter than the bearing ball 20, into thetriangular recess.

The recesses may be coated with friction reducing material. The ballsmay be coated with friction reducing material also, but may lose some oftheir sphericity in the process, so this step is not favoured.

FIG. 3 is a section along line III—III in FIG. 2. In this drawing thesupport 28 is shown in a loaded condition. In this condition support 28is deflected and each area 36 will have moved from an unloaded conditionby the same amount relative to the axis of rotation. It will be notedthat the curvature of the surface of each area 36 provides a contactpoint 42. In order to make this contact point lie in the middle of area36, the surface is formed with its radius when the support 28 is loaded.A load may be applied to the support during assembly as a preload.

A cone generated by the revolution of a line between the centre of theball 20 and its points of contact 42 with the support 28 will have acone angle θ. This angle θ is chosen to minimise rotational friction, tomaintain bearing radial stiffness and to inhibit dislodgement of theball. Any angle between 20° and 175° can be used but an angle of about800 has been found to be suitable.

The ball contacting areas 36 may be formed by forcing a cone into asupport rather than a ball, to produce partially conical contact areas36. In this instance, point contact will be achieved also.

FIG. 4 shows a detail of a fixed support 29. This support may bemanufactured in a slightly different manner to the support 28 shown inFIG. 3. Support 29 is not deflected in use but is held comparativelyrigidly by lower housing 12. The contact surface 36 is again formed witha slightly larger radius than the radius of the ball but the support isnot deflected whilst the radius is formed. The cone angle θ may beapproximately the same as the angle θ formed at support 28.

Referring back to FIG. 1, spindle 18 is mounted between supports 30 and32. An extension 19 of the spindle 18 carries the lower housing 12. Theprinciples of construction of the balls 24,26 and their supports 30,32are the same as those described above for the balls 20,22 and theirsupports 28,29. However, ball 24 is larger than the others because itcarries the weight of the lower housing 12 and so requires greaterstiffness, also it has to accommodate spindle extension 19. In thisarrangement support 30 is fixed and support 32 is deflectable. Support32 will be preloaded in the manner described above. The cone angles willbe similar. Use of extension 19 through bearing 24/30 together with therotary coupling 90 allows continuous rotations of lower housing 12 andits associated parts.

The configuration of spindle 18 and supports 30 and 32 providestolerance to thermal effects because the lower end of the CMM quill 8 isclose to the plane in which support 30 and ball 24 have contact. Thusthermal growth of housing 10 does not alter significantly the positionof housing 12 because there is little material in the width between thebase of the CMM quill and the said plane.

This means that any changes in dimension of upper housing 10 due tothermal effects will not alter substantially the position of the stylus4.

An alternative bearing arrangement for upper housing 10 is shown in FIG.5. This arrangement has a fixed support 30 at the top and thedeflectable support 32 below.

FIGS. 6A-6F show examples of some permutations of ball and supportconfiguration. For ease of identification the fixed supports have beenreferenced as 50, deflectable supports 55, and relatively rotatableparts 60 and 65. All these permutations fall within the ambit of theinvention.

FIG. 7 shows another, alternative arrangement of bearing parts. In thisarrangement balls 20 and 22 are each supported in ball race bearings 80.The races 80 are in turn held in supports 281 and 291. As discussedabove support 28′ allows some axial movement, and support 291 isrelatively rigid. A preload is applied to the supports and balls asdiscussed above. Use of ball races 80 reduces friction. The supports 28′and 29′ need not have triangular holes, holding the ball races 80 inplace.

The reference numerals used in FIG. 7 relate to the horizontal axisshown in FIG. 1, however, a similar “ball raced” construction may beemployed also for the vertical axis bearings of the embodiment shown inFIG. 1.

An alternative construction of the ball and support contact area isshown in FIG. 8. In this embodiment support 28,29,30,32 is flanged toaccommodate ball 20,22,24,26. The ball rests on an annular low frictioncollar 82 which may be manufactured from P.T.F.E., a ceramic or asimilar low friction material. The collar may be replaced with threepads of a similar material. In either case the collar or pads may befixed with adhesive to the support.

Yet another alternative construction of the ball and support contactarea is shown in FIG. 9. In this embodiment support 28,29,30,32 hasadhered thereon a block of elastically deformable material e.g. P.T.F.E.or other low friction plastics material or an oil impregnated sinteredproduct having a conical recess therein. The recess accommodates ball20,22,24,26. The elastic properties of the material of the block takesup slight deviations in the roundness of the ball so that the relativerotation of the block and ball has greater circular accuracy than thecircular accuracy of the ball alone.

Further embodiments of the ball and support construction are shown inFIGS. 10 and 11. In each of the Figures ball 20,22,24,26 is supported ina plastics insert 85 of P.T.F.E. or the like supported within an outercollar 90. The ball of FIG. 10 is supported in a cylindrical recess 92and the ball of FIG. 11 is supported in a conical recess 94. Again theplastics insert will, under load, deform elastically to the shape of theball and during relative rotation take up any out-of-roundness of theball to give accurate circular motion.

Various modifications and enhancements will be apparent to the skilledaddressee. For example whilst spherical bearings and triangularapertures are illustrated, within the ambit of the invention, anyconfiguration of sliding contact between male and female parts ispossible providing contact is made in a plane. It is possible that thisplane, whilst it will be perpendicular to the axis of rotation in theillustrated embodiments, may not be so e.g. if supports 28 and 29 saylie in planes offset to the perpendicular and balls 20 and 22 are used.

So the supports illustrated may, according to the invention, be simply arecess e.g. a circular hole, a conical recess, a trihedral recess, amulti-sided hole, or the like, perhaps formed directly into a housing 12or 10, or in the form of an insert in a sprung plate. A support might beformed also in a spindle 16 or 18, in which case a male part would beconnected to the housing 10 or 12.

The male part may be any shape e.g. spheroidal, elliptical, conical,trihedral, or polygonal. If the male part is non-circular then thesupport must be circular, and vice versa. Where the support or male partis non-circular then contact at discrete points between the two is made,these points will lie on a plane also.

The male/female parts of each bearing assembly may differ inconstruction.

Whilst the discrete locations at which the balls make sliding contactwith the supports are illustrated as sides of an aperture, they may bethe sides of a recess, e.g. a low friction insert having a facetedrecess in a support. Contact might be provided at the edges of anaperture or recess.

Also the spindles 16 and 18 may be constrained against excessivemovement away from their axes of rotation such that should the spindlebe forced out of register with a support it would be pushed back intoregister by virtue of the resilient nature of one of the supports.

The embodiments described above have certain advantages over the priorart. In particular, the embodiments allow wear and/or changes indimensions of the component parts mentioned to be accommodated withoutsignificant loss in accuracy or repeatability, i.e. change in positionof the stylus.

For example a change in the dimension between balls 20 and 22 can beaccommodated by resilient movement of support 28. This movement causesall points of contact between support 28 and ball 20 to move along theaxis A by the same amount and therefore the position of the axis A doesnot change as a result of the resilient movement. Whilst the stylus maymove slightly parallel to the axis A, its movement will be small incomparison to the change in dimension between the walls 20 and 22(approximately half).

Positional stability of the stylus is achieved also by the provision ofa fixed support 29. The fixed support provides positive positioning ofthe spindle 16 at one position on the axis A.

The same advantages are gained by the configuration of the bearings30/24 and 32/26 in the upper housing 10. Likewise the permutations shownin FIGS. 6 a-f have the advantages mentioned in the preceding paragraph.

1. A bearing arrangement comprising: two bearing assemblies both locatedon the same axis; each bearing assembly comprising two parts including afemale part having a recess or aperture and a male part acceptable intothe recess or aperture in sliding contact during their relativerotation; at each assembly the sliding contact taking place in arespective plane at a plurality of discrete locations on the male orfemale parts; wherein one of the assemblies allows resilientdisplacement of its contact plane and the other of the assemblies isrelatively rigid for preventing substantial displacement of its contactplane.
 2. A bearing arrangement as claimed in claim 1 wherein thedisplacement of the contact plane is allowed to take place only in adirection substantially parallel to the axis.
 3. A bearing arrangementas claimed in claim 1 wherein the discrete locations are provided by anon-circular recess or aperture in the female part co-operating with acircular male part, or are provided by a circular recess or aperture inthe female part co-operating with a non-circular male part.
 4. A bearingarrangement as claimed in claim 3 wherein the non-circular recess oraperture is triangular or trihedral and the circular male part isspheroidal or conical.
 5. A bearing arrangement as claimed in claim 3wherein the non-circular male part is trihedral.
 6. A bearingarrangement as claimed in claim 1 wherein at least one of the two partsis formed of a plastics material.
 7. A bearing arrangement as claimed inclaim 6 wherein the plastics material is formed as an insert within anouter collar.
 8. A bearing arrangement as claimed in claim 1 wherein theresilient displacement of the plane is provided by a resiliently movablefemale part.
 9. A bearing arrangement as claimed in claim 8 wherein thefemale part includes a planar spring support.
 10. A measurement probesupport having a pivot including a bearing arrangement according toclaim
 1. 11. A support for a measurement probe comprising anarticulatable wrist providing two axes of rotation for the probe, at afirst axis there being provided a first bearing arrangement as claimedin claim 1, the first bearing arrangement being connected to a spindlehaving an extension extending beyond the first bearing arrangement inthe direction of the first axis.
 12. A support for a measurement probeas claimed in claim 11 wherein the extension is connected to a secondbearing arrangement providing a second axis of rotation for the probe,transverse to the first axis.
 13. A support for a measurement probe asclaimed in claim 12 wherein power and signal paths are provided and atleast one of the paths crosses a rotary coupling disposed about thefirst axis.
 14. A support for a measurement probe as claimed in claim 11wherein the first bearing arrangement comprises a ball in a recess andthe extension extends beyond the ball.
 15. A bearing arrangementcomprising: two bearing assemblies both located on the same axis; eachbearing assembly comprising two parts including a female part having arecess or aperture and a male part acceptable into the recess oraperture in sliding contact during their relative rotation; at eachassembly the sliding contact taking place in a respective plane at aplurality of discrete locations on the male or female parts; wherein atleast one of the assemblies allows resilient displacement of its contactplane in a direction parallel to the axis.